1,745 research outputs found
A study of ignition phenomena of bulk metals by radiant heating
Early research on combustion of metals was motivated by the knowledge of the large heat release and corresponding high temperatures associated with metal-oxygen reactions. The advent of space flight brought about an increased interest in the ignition and combustion of metallic particles as additives in solid rocket propellants. More recently, attention has been given to the flammability properties of bulk, structural metals due to the number of accidental explosions of metal components in high-pressure oxygen systems. The following work represents a preliminary study that is part of a broader research effort aimed at providing further insight into the phenomena of bulk metal combustion by looking at the effects of gravity on the ignition behavior of metals. The scope of this preliminary experimental study includes the use of a non-coherent, continuous radiation ignition source, the measurement of temperature profiles of a variety of metals and a qualitative observation of the ignition phenomena at normal gravity. The specific objectives of the investigation include: (1) a feasibility study of the use of a continuous radiation source for metal ignition; (2) testing and characterization of the ignition behavior of a variety of metals; and (3) building a preliminary experimental database on ignition of metals under normal gravity conditions
C-axis Raman spectra of a normal plane-chain bilayer cuprate and the pseudogap
We investigate the Raman spectra in the geometry where both incident and
scattered photon polarizations are parallel to the -direction, for a
plane-chain bilayer coupled via a single-particle tunneling . The
Raman vertex is derived in the tight-binding limit and in the absence of
Coulomb screening, the Raman intensity can be separated into intraband
() and interband () transitions. In the
small- limit, the interband part dominates and a pseudogap will appear
as it does in the conductivity. Coulomb interactions bring in a two-particle
coupling and result in the breakdown of intra- and interband separation.
Nevertheless, when is small, the Coulomb screening () has little effect on the intensity to which the unscreened
interband transitions contribute most. In general, the total Raman spectra are
strongly dependent on the magnitude of .Comment: 23 pages, 6 figures, submitted to Phys. Rev.
Antarctic killer whales make rapid, round-trip movements to subtropical waters: evidence for physiological maintenance migrations?
Killer whales (Orcinus orca) are important predators in high latitudes, where their ecological impact is mediated through their movements. We used satellite telemetry to provide the first evidence of migration for killer whales, characterized by fast (more than 12 km hâ1, 6.5 knots) and direct movements away from Antarctic waters by six of 12 type B killer whales tagged when foraging near the Antarctic Peninsula, including all tags transmitting for more than three weeks. Tags on five of these whales revealed consistent movements to subtropical waters (30â37° S) off Uruguay and Brazil, in surface water temperatures ranging from â1.9°C to 24.2°C; one 109 day track documented a non-stop round trip of almost 9400 km (5075 nmi) in just 42 days. Although whales travelled slower in the warmest waters, there was no obvious interruption in swim speed or direction to indicate calving or prolonged feeding. Furthermore, these movements were aseasonal, initiating over 80 days between February and April; one whale returned to within 40 km of the tagging site at the onset of the austral winter in June. We suggest that these movements may represent periodic maintenance migrations, with warmer waters allowing skin regeneration without the high cost of heat loss: a physiological constraint that may also affect other whales
Electronic Raman scattering in YBCO and other superconducting cuprates
Superconductivity induced structures in the electronic Raman spectra of
high-Tc superconductors are computed using the results of ab initio LDA-LMTO
three-dimensional band structure calculations via numerical integrations of the
mass fluctuations, either in the whole 3D Brillouin zone or limiting the
integrations to the Fermi surface. The results of both calculations are rather
similar, the Brillouin zone integration yielding additional weak structures
related to the extended van Hove singularities. Similar calculations have been
performed for the normal state of these high-Tc cuprates. Polarization
configurations have been investigated and the results have been compared to
experimental spectra. The assumption of a simple d_(x^2-y^2)-like gap function
allows us to explain a number of experimental features but is hard to reconcile
with the relative positions of the A1g and B1g peaks.Comment: 14 pages, LaTeX (RevTeX), 5 PostScript figures, uses multicol.sty,
submitted to PR
Conformational spread as a mechanism for cooperativity in the bacterial flagellar switch
The bacterial flagellar switch that controls the direction of flagellar rotation during chemotaxis has a highly cooperative response. This has previously been understood in terms of the classic two-state, concerted model of allosteric regulation. Here, we used high-resolution optical microscopy to observe switching of single motors and uncover the stochastic multistate nature of the switch. Our observations are in detailed quantitative agreement with a recent general model of allosteric cooperativity that exhibits conformational spreadâthe stochastic growth and shrinkage of domains of adjacent subunits sharing a particular conformational state. We expect that conformational spread will be important in explaining cooperativity in other large signaling complexes
Modeling the radial abundance distribution of the transition galaxy ngc 1313
NGC 1313 is the most massive disk galaxy showing a flat radial abundance
distribution in its interstellar gas, a behavior generally observed in
magellanic and irregular galaxies. We have attempted to reproduce this flat
abundance distribution using a multiphase chemical evolution model, which has
been previously used sucessfully to depict other spiral galaxies along the
Hubble morphological sequence. We found that it is not possible to reproduce
the flat radial abundance distribution in NGC 1313, and at the same time, be
consistent with observed radial distributions of other key parameters such the
surface gas density and star formation profiles. We conclude that a more
complicated galactic evolution model including radial flows, and possibly mass
loss due to supernova explosions and winds, is necessary to explain the
apparent chemical uniformity of the disk of NGC 1313Comment: 14 paginas, 4 figures, to be published in ApJ, apri
Type Ia Supernovae and Cosmology
I discuss the use of Type Ia supernovae (SNe Ia) for cosmological distance
determinations. Low-redshift SNe Ia (z < 0.1) demonstrate that the Hubble
expansion is linear with H_0 = 72 +/- 8 km/s/Mpc, and that the properties of
dust in other galaxies are generally similar to those of dust in the Milky Way.
The measured luminosity distances of SNe Ia as a function of redshift have
shown that the expansion of the Universe is currently accelerating, probably
due to the presence of repulsive dark energy such as Einstein's cosmological
constant (Lambda). From about 200 SNe Ia, we find that Omega_Lambda - 1.4
Omega_M = 0.35 +/- 0.14. Combining our data with other results, we find a best
fit for Omega_M and Omega_Lambda of 0.28 and 0.72, respectively. A number of
possible systematic effects (dust, supernova evolution) thus far do not seem to
eliminate the need for Omega_Lambda > 0. Recently, analyses of SNe Ia at z =
1.0-1.7 provide further support for current acceleration, and give tentative
evidence for an early epoch of deceleration. The dynamical age of the Universe
is estimated to be 13.1 +/- 1.5 Gyr. According to the most recent data sets,
the SN Ia rate at z > 1 is several times greater than that at low redshifts,
presumably because of higher star formation rates long ago. Moreover, the
typical delay time from progenitor star formation to SNIa explosion appears to
be substantial, ~3 Gyr. Current projects include the measurement of a few
hundred SNe Ia at z = 0.2-0.8 to more accurately determine the
equation-of-state parameter of the dark energy, w = P/(\rho c^2), whose value
is now constrained by SNe Ia to be in the range -1.48 < w < -0.72 at 95%
confidence.Comment: 39 pages, 17 figures, to be published in "White Dwarfs: Probes of
Galactic Structure and Cosmology" ed. E. M. Sion, H. L. Shipman, and S.
Vennes (Kluwer: Dordrecht). Part of the Astrophysics and Space Science
Library Serie
Geocentrism reexamined
The universe is nearly isotropic on very large scales. It is much more
difficult to show that the universe is radially homogeneous (independent of
distance), or equivalently, that it is isotropic about distant points. This
taken as an axiom, since if it were not true, then we would occupy a preferred
position. This paper considers several empirical arguments for radial
homogeneity based on the cosmic microwave background (CMB). The tightest limits
on inhomogeneity on the scale of the horizon are of order ten percent but will
improve soon. These limits involve the Sunyaev-Zel'dovich effect in clusters of
galaxies, excitation of low-energy atomic transitions, and the accurately
thermal spectrum of the CMB. Weaker limits from primordial nucleosynthesis are
discussed briefly.Comment: RevTeX source, 14 pages, no figs. To appear Phys Rev
- âŠ